In an optical transmission system using the digital coherent detection technology, it is important to improve an optical signal-to-noise ratio (OSNR). By improving the OSNR, it is possible to increase transmission capacity of an optical transmission system and to increase a transmittable distance and a repeater interval (span length).
To improve the OSNR, it is effective to reduce transmission loss of a transmission optical fiber, and to decrease nonlinearity by increasing an effective area Aeff of the optical fiber so as to reduce optical power density in the core.
Standard single-mode optical fiber compliant to ITU-T G.652 recommendation has a transmission loss of approximately 0.19 dB/km at a wavelength of 1550 nm and Aeff of approximately 80 μm2 at a wavelength of 1550 nm. Compared with this fiber, the optical fiber described in Patent Literature 1 has a transmission loss of 0.18 dB/km or less at a wavelength of 1550 nm and Aeff of 110 μm2 or more at a wavelength of 1550 nm.
In general, in an optical transmission system, optical fibers are deployed between a transmitter and the first stage repeater, between a repeater and the next stage repeater, and the final stage repeater and a receiver, and transmit an optical signal from the transmitter to the receiver. Typically, the optical signal is amplified by an Er-doped optical fiber amplifier (EDFA) disposed in a repeater so that transmission loss caused between repeaters (one span) is completely compensated.
On the other hand, in order to improve the OSNR of the optical transmission system, Raman amplification, which is excellent in noise characteristics, is used in combination with an EDFA in some cases. FIG. 1A is a diagram showing a configuration of an optical transmission system 1 in which Raman amplification and the EDFA are used in combination. The optical transmission system 1 includes a transmitter 10, a repeater 20, and a receiver 30, and between these devices are deployed optical fibers 40 as optical fiber transmission lines. Each repeater 20 includes an EDFA 21 and a pumping light source 22 for Raman amplification. A Raman amplifier comprises the pumping light source 22 for Raman amplification in each repeater 20 and the optical fiber 40 through which pumping light for Raman amplification emitted from the light source 22 for Raman amplification propagates.
The light source 22 for Raman amplification in each repeater 20 amplifies signal light by causing the pumping light to propagate in the same or opposite direction of the propagation direction of the signal light and thus to create stimulated Raman scattering in the transmission optical fiber 40. A gain of the Raman amplification in an optical fiber made of silica-based glass has a peak at a wavelength which is approximately 100 nm longer than the pumping light wavelength. For example, in order to Raman-amplify the signal light is in the C band (wavelength of 1530 nm to 1565 nm), the pumping light wavelength is set near a wavelength of 1450 nm in many cases.
However, because Raman amplification uses stimulated Raman scattering, which is one of nonlinear phenomena, there is a problem that amplification efficiency decreases as Aeff is increased.
Patent Literature 2 describes an optical fiber and an optical transmission system which improve OSNR in a wavelength division multiplexed optical transmission system which uses Raman amplification and is for a long repeating distance. Further, this document describes that Aeff at a wavelength of 1550 nm must be 150 μm2 or less in order to keep the pumping light power in a practical range (some watts or less).
Patent Literature 3 describes the relationship between transmission loss and Aeff at a wavelength of 1550 nm, and the relationship needs to be satisfied to improve the OSNR and at the same time to suppress increase in power of pumping light for Raman amplification in the optical transmission system using Raman amplification.